Currently, magnetoresistive (MR) heads are currently used in read heads or for reading in a composite head. MR heads use an MR sensor in order to read data that has been stored in magnetic recording media. Giant magnetoresistance ("GMR") has been found to provide a higher signal for a given magnetic field. Thus, GMR is increasingly used as a mechanism for higher density MR sensors. One MR sensor which utilizes GMR to sense the magnetization stored in recording media is a conventional spin valve. A conventional spin valve includes two magnetic layers, a free layer and a pinned layer, a spacer layer, and a conventional antiferromagnetic (AFM) layer. The conventional spin valve may also include a capping layer. The spacer layer separates the free layer from the pinned layer. The magnetization of the pinned layer is typically fixed by exchange coupling to the conventional AFM layer.
More recently, conventional dual spin valves have been developed. For example, a conventional dual spin valve is disclosed in U.S. Pat. No. 5,287,238. A conventional dual spin valve typically includes a first AFM layer, a pinned layer on the first AFM layer, a spacer layer, a free layer on the spacer layer, a second pinned layer on the free layer, and a second AFM layer on the second pinned layer. The pinned layers and the free layer are still magnetic layers. The magnetization of the first and second pinned layers is fixed by an exchange coupling with the first and second AFM layers, respectively. Because there are more interfaces between the spacer layers and the magnetic (pinned and free) layers, the conventional dual spin valve has more scattering surfaces. As a result, the conventional dual spin valve has a higher MR.
In order to use the conventional spin valve or the conventional dual spin valve as a conventional MR sensor, current is passed through the conventional MR sensor as the MR head is brought in proximity to a recording media. Based on the information stored in the recording media, the resistance of the conventional MR sensor can change. Thus, the conventional MR sensor can be used to read the data stored by the recording media.
Because the conventional spin valve has a higher MR than anisotropic MR sensors, it has a higher signal. A conventional spin valve is preferred over anistropic MR sensors for higher densities. The conventional dual spin valve has an even higher MR than the conventional spin valve. Thus, the conventional dual spin valve is preferred for many applications.
Although the conventional spin valve and conventional dual spin valves are usable, there are drawbacks to each. The magnetizations of the pinned layers in both the conventional spin valve and the conventional dual spin valve are pinned due to a magnetic coupling with the antiferromagnetic layers. This coupling between the antiferromagnetic layers and the pinned layers is stable only up to approximately the blocking temperature of the antiferromagnetic material used. Above this temperature, the antiferromagnetic layers cannot effectively pin the magnetizations of the pinned layers. Thus, from approximately the blocking temperature and above, the conventional spin valve and conventional dual are not stable. Thus, the range of temperatures at which the conventional spin valve and conventional dual spin valve is limited.
Furthermore, the magnetizations of the pinned layers in the conventional spin valve and the conventional dual spin valve create a magnetostatic field in the region of the free layer. This field is present even when no current is passed through the MR sensor. When current is passed through a conventional spin valve, the magnetic field at the free layer changes based on the current. Although the current driven through the conventional spin valve can offset the pinned layer's magnetic field at the free layer, this offset occurs only at a particular current. If the current varies significantly from the proper current, the response of the conventional spin valve due to external magnetic fields will not be symmetric around a zero external magnetic field. The pinned layer thickness and the current must be optimized in order to produce symmetric behavior near the optimal current. In a conventional dual spin valve, the field generated by the current does not change the magnetic field at the free layer. There is, however, magnetic field present at the free layer due to the pinned layers. Thus, the MR and the signal of the conventional dual spin valve are also not symmetric.
Accordingly, what is needed is a system and method for providing a thermally stable MR head. It would also be desirable for the MR head to be symmetric and have a wide linear range. The present invention addresses such a need.